Progenitor cells capable of hematopoietic reconstitution after myeloablative therapy have been identified in a number of sources including the bone marrow, umbilical cord and placental blood, and in the peripheral blood of subjects treated with stem cell-mobilizing doses of granulocyte-colony stimulation factor. These cells, often referred to as hematopoietic stem cells (HSC), are identified by the presence of cell surface glycoproteins such as CD34 and CD133. HSC represent a very small percentage of the total population of cells given as part of a ‘bone marrow transplant’ and are considered to be the life-saving therapeutic portion of this treatment responsible for the restoration of the blood-forming capacity of patients given myeloablative doses of chemotherapy or radiation therapy. Stem cell therapies via bone marrow transplantation have become a standard treatment for a number of intractable leukemias and genetic blood disorders.
Recent studies have suggested the presence of a more primitive cell population in the bone marrow capable of self-renewal as well as differentiation into a number of different tissue types other than blood cells. These multi-potential cells were discovered as a minor component in the CD34-plastic-adherent cell population of adult bone marrow, and are variously referred to as mesenchymal stem cells (MSC) (Pittenger, et al., Science 284:143-147 (1999)) or multi-potent adult progenitor cells (MAPC) cells (Furcht, L. T., et al., U.S. patent publication 20040107453 A1). MSC cells do not have a single specific identifying marker, but have been shown to be positive for a number of markers, including CD29, CD90, CD105, and CD73, and negative for other markers, including CD14, CD3, and CD34. Various groups have reported to differentiate MSC cells into myocytes, neurons, pancreatic beta-cells, liver cells, bone cells, and connective tissue. Another group (Wernet et al., U.S. patent publication 20020164794 A1) has described an unrestricted somatic stem cell (USSC) with multi-potential capacity that is derived from a CD45−/CD34− population within cord blood.